The linear induction motor (hereafter LIM) as classically practiced generates a linear motion by inductive coupling between a primary or stator which creates a linearly moving magnetic field and a conductive secondary or armature in which thrust is developed by virtue of an induced field causing mutual repulsion between the primary and secondary. The primary is usually but not necessarily the stationary member, and the system can be single sided or double sided, with primaries on opposite sides of the secondary. The linear motor seeks to generate a velocity in the moving element that approaches synchronism with the input wave that actuates the primary. Accelerations and velocities are limited to less than the theoretical maximums by slip, friction, drag and load factors. Velocity and position control schemes for linear motors usually have employed some form of signal control to modulate frequency or power so as to vary thrust in a predetermined way determinative of the desired velocity profile. These control approaches are inherently complex, however, particularly when it is desirable to control a number of different members simultaneously with the same linear propulsion device. In addition, because of their mode of operation it has been difficult to operate linear induction motors at slow speeds and bidirectionally. There are many applications in which the other advantages of LIMs can provide unique benefits if these capabilities can also be supplied. In automated material conveyor systems, for example, a rail or track structure is extended between material pick up stations, delivery and/or work stations. In general, a number of carriers or carts are disposed on the rails of the conveyor system and a drive system extending along the path of the conveyor system serves to move each cart from station to station. In one widely used form of this system, power is transmitted to individual carriers from an elongated rotating member extending along the conveyor path. Each carrier incorporates a controllable power takeoff that is driven frictionally by the rotating member. The angle of the power takeoff determines the velocity of the carrier, and this angle is controlled at each position along the path by an extended cam which roughly parallels the carrier track. This system is not only costly and has inherent frictional wear problems but top speed is limited by the length of the rotating member.
Another proposed system for material conveyors, as in U.S. Pat. No. 3,641,939, describes the use of a LIM having a speed regulating arrangement using a movable secondary on each truck or carrier. The secondary is movable toward and away from a primary so as to alter the inductive coupling between them and thereby assertedly control the velocity. Although various arrangements are shown, including a moving secondary that shifts with the weight of a load and single and double ended versions in which the spacings are varied by a cam surface, no such system has appeared in a commercial version. It is likely that there are several reasons the system was not successful. Varying the gap spacing between primary and secondary requires significant force, and at the same time substantially changes the coupling efficiency in non-linear fashion. Thrust cannot be reversed for individual carriers except by complex controls, and slow speed operation and freedom from creep also cannot be attained. The technology still seeks a system that enables high speed, efficient movement of a carrier between successive work stations with controlled acceleration and deceleration, and stable positioning at index locations.